The present invention relates generally to radiation detection and, more particularly, to an improved apparatus and method of light collection for use with a radiation emitting medical imaging scanner.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward an object, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam of radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the object. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately results in the formation of an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the object. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator.
Typically, the scintillator or scintillation detector used with CT systems and other radiation emitting medical imaging scanners are coupled to a light collection device such as a photodiode using a transparent adhesive. The transparent adhesive, however, typically limits the angles at which light can exit the scintillator to those greater than the critical angle of the interface between the scintillator and the adhesive. Moreover, scintillators usually have very high indices of refraction due to the high density required to prevent radiation leakage. As a result, an appreciable index change at the scintillator exit face occurs with resulting total internal reflection of light striking the exit face at relatively shallow angles.
Further, these known systems typically implement a rectangular block shaped scintillator to stop the radiation and maximize the area exposed. While this rectangular shape accommodates an easy to fabricate area-filling shape with the appropriate thickness for optimum radiation stopping power, the rectangular shape has several disadvantages associated with light collection by a photodiode.
For example, parallel plane walls are typically implemented that are perpendicular to the light detector photodiode thereby causing light that is emitted nearly parallel to the light collector face to bounce repeatedly off the parallel walls either by specular reflection or by total internal reflection. Moreover, there is no means of directing light preferentially toward the light detector in these rectangular shaped scintillators. Diffused reflectors have been implemented to randomize the direction of light rays, but these diffused reflectors result in the need for thicker reflectors to provide sufficient scatter by light refraction. As a result, these thicker reflectors decrease the possible re-fraction of the scintillator. Additionally, these reflectors are also not opaque so an additional opaque component to stop light leakage between cells must be employed. Other scintillators have been designed using a defused opaque coating, but the increased area of such a coating and the opportunity for multiple bounces typically results in a considerable decrease and reflectivity compared to a specular coating.
Additional disadvantages often associated with these rectangular shaped scintillators include the reflection of light back into the scintillator when the light hits the exit surface of the scintillator at an angle greater than the critical angle of the scintillator-adhesive interface. This disadvantage, as well as the previously discussed disadvantages, increase the mean number of reflections that light undergoes before striking the detector. Simply, each reflection off the side reflector of the scintillators results in a loss from the absorbance of the reflector.
It would therefore be desirable to design an apparatus and method of light collection whereby light throughput of a scintillator is increased and light collection efficiency likewise improves.
The present invention is directed to improved scintillator detector cell geometry overcoming the aforementioned drawbacks. Shaping the exit face of a scintillator to increase the surface area results in a decrease in a fraction of angles that undergo total internal reflection within the scintillator. By convexly shaping the exit face, the scintillator has the advantage of preventing total internal reflection parallel, as well as perpendicular, to the detecting surface of a light collection device. Further, providing a specular reflector on a hemispherical dome portion of the radiation detecting surface of the scintillator results in reduced reflection off the specular reflector before light contacts the scintillator-photodiode interface. Furthermore, implementing a convex shape that is coated with the specular reflector, increases the fraction of light directed toward the photodiode compared to a planar surface parallel to the photodiode. The present invention further limits the amount of light that is trapped within the scintillator.
Therefore, in accordance with one aspect of the present invention, a scintillation apparatus for use with a radiation emitting medical imaging scanner is provided. The scintillation apparatus includes an entrance face configured to receive radiation and an exit face having a tetrahedral shape and configured to discharge light. The scintillation apparatus further includes a plurality of plane walls extending from the entrance face to the exit face.
In accordance with another aspect of the present invention, a CT system includes a scintillator array having a plurality of scintillation cells. Each scintillation cell of the CT system has at least one of a non-planar radiation reception surface and a non-planar light emitting surface. The non-planar reception surface and the non-planar light emitting surface are symmetrically shaped with respect to one another. The CT system further includes a radiation projection source configured to project radiation toward the scintillator array and a photodiode array having a plurality of photodiodes. The photodiode array is optically coupled to the scintillator array to detect light output therefrom. The CT system further includes a gantry having an opening to receive a subject to be scanned.
In accordance with yet another aspect of the present invention, a radiation detector for use with the radiation emitting medical imaging scanner is provided. The radiation detector includes a means for detecting radiation as well as a means for converting the radiation to light energy. The radiation detector further includes a means for emitting light energy toward a light energy detector and a means for reducing light energy bounce off within the radiation detector.
In accordance with a further aspect of the present invention, a method of light collection from a scintillation detector of a radiation emitting medical imaging scanner includes directing radiation toward a scan subject and a scintillation detector. The method further includes receiving radiation attenuated by the scan subject and converting the attenuated radiation to light energy. The light energy is then admitted through a non-planar surface of the scintillation detector. The method further includes detecting the admitted light energy from the scintillation detector.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.